The sum of $2$ consecutive integers is $21$. What is the first number in this sequence?
Solution: Call the first number in the sequence $x$ The next integer in the sequence is $x + 1$ The sum of the $2$ consecutive integers is: $x+ (x + 1) = 21$ $2x + 1= 21$ $2x = 20$ $x = 10$ Thus, the first number is $10$.